1. Field of the Invention
The present invention relates generally to a rubber composition for an inner liner of a tire, and more particularly to improvements in an inner liner for a tire whose internal pressure is required to be securely maintained as particularly in radial-ply and bias-ply tires for passenger cars, tires for motorcycles, and tires for trucks and buses.
2. Description of the Prior Art
In connection with radial-ply and bias-ply tires of passenger cars, tires of motorcycles, and tires of trucks and buses, it is important to maintain the internal pressure of the tires. For this purpose, a rubber composition including as a main component a halogenated butylene rubber has been hitherto used as the inner liner of the tire as disclosed, for example, in published Japanese Patent Application Nos. 57-172945 and 57-195760.
Such a rubber composition including as the main component halogenated butylene rubber is useful from a view point of maintaining the internal pressure of tires. However, the rubber composition is inferior in stickiness due to the fact that its main component is halogenated butylene rubber, so that a sufficiently tight contact cannot be obtained between the inner liner and a tire casing during fabrication of a green tire. This will unavoidably allow air to enter between the inner liner and the tire casing after vulcanization. Additionally, a sufficient adhesive strength cannot be obtained between the inner liner and the tire casing, thereby causing peeling of the inner liner layer. Furthermore, because of insufficiently tight contact of the inner liner layer with the tire casing, lubricant and the like applied before vulcanization for the purpose of tight contact with a vulcanization bladder unavoidably enters to between the inner liner and the tire casing, so that a sufficient adhesive strength cannot be obtained between the inner liner layer and the tire casing. This will allow a crack to be formed or advanced where the lubricant and the like has entered, thereby causing breakdown of the inner liner and the tire casing.
In this regard, it is usual to blend a suitable amount of tackifier in order to increase the stickiness of the rubber composition of the inner liner layer. Examples of said tackifiers are phenol resins, terpene resins, and petroleum hydrocarbon resins. However, these resins unavoidably harden at low temperatures and therefore adversely affect the low-temperature characteristics of inner liners containing such resins, thereby permitting cracks to form in the inner liner when the vehicle is cruising at low temperatures. In view of this, blending a suitable amount of softener has been conventionally employed to improve the low-temperature characteristics. However, even with this measure, it has been difficult to maintain a proper balance between the desired high level of stickiness before vulcanization, and crack formation preventing characteristics during low temperature vehicle cruising and during normal vehicle cruising.